1. Field of the Invention
The present invention relates to a photo mask for use in a semiconductor field, a focus measuring method using the mask, and a method of manufacturing a semiconductor device.
2. Description of the Related Art
A tolerance of focus permitted in lithography has been narrowed as a design rule of a semiconductor device to be manufactured is miniaturized. When the tolerance of the focus is narrowed, flatness of a wafer and specifications with respect to curvature of field of an exposure apparatus has been strict. Moreover, a high-precision measuring method of the focus, curvature of field and the like using a resist pattern transferred onto the wafer has become important.
A focus test mask comprising an asymmetrical diffraction grating pattern and a reference pattern, and a focus measuring method using the focus test mask and utilizing a phenomenon in which an image of the asymmetrical diffraction grating pattern shifts in proportion to a focus value have been known (Jpn. Pat. No. 3297423). Since the focus measuring method has a high measurement precision having a measurement error of 5 nm or less, and the measuring is simple, the method can be said to be one of most promising techniques at present.
The asymmetrical diffraction grating pattern comprises a shielding portion, a transmitting portion, and 90° phase grooved portion. A line width ratio of the shielding portion, transmitting portion, and 90° phase grooved portion is ideally 2:1:1. On the other hand, an alternating type phase shift exposure mask including a pattern (device pattern) for manufacturing an actual semiconductor product comprises a 180° phase grooved portion.
A method of manufacturing an exposure mask comprising the asymmetrical diffraction grating pattern and the device pattern includes a step of forming the 90° phase grooved portion, and a step of forming the 180° phase grooved portion. When these two steps are performed, a manufacturing process is complicated, and manufacturing costs remarkably rise. This respect will be further described hereinafter.
The step of forming the 180° phase grooved portion includes a step of forming a trench vertically in the surface of a quartz glass substrate by a dry process (e.g., a vertical etching process such as an RIE process); and a step of expanding the trench by predetermined amounts in a lateral direction and a vertical direction by a wet process (isotropic etching process). A sum of grooved amounts by the dry and wet processes is a grooved amount by which a phase of transmitted light delays by 180° as compared with a case where there is not any grooved portion.
To obtain a high-precision alternating type phase shift exposure mask, the groove has to be made vertically, and further expanded in the lateral direction. However, an etching process to expand the groove only in the lateral direction does not exist. Therefore, as described above, combined use of the dry and wet processes is required. Since an amount to be expanded in the lateral direction needs to be controlled with a high precision, the grooved amount in the dry process is a depth obtained by subtracting the amount to be expanded in the lateral direction from the amount corresponding to 180°. On the other hand, to obtain a high-precision focus test mask, a grooved portion corresponding to 90° has to be formed only by the dry process.
Therefore, in a conventional technique, the above-described etching processes have to be separately performed in order to realize the high-precision alternating type phase shift exposure mask and the high-precision focus test mask in one exposure mask, and a mask manufacturing cost rises by at least 30% or more as compared with the conventional alternating type phase shift exposure mask.